Hi,
I sort of figured it out. It seems as if new splash pages now have to be made within Configuration > Networks >. However, I can still not edit a shared splash page made on MSP level.
And yes, there is a valid Guest subscription on the AP's for the customers in question.
What I meant with difficulty with support is mainly for submitting a case online on the HPE Networking site as it requires a serial number or product code to go any steps further.
I have refrained from calling so far, but I will keep it in mind for later use. Thanks :)